Let me start by stating that I may be doing something wrong (there's not much documentation on this product so far...).

I designed my entire model from scratch in SDDM 3, and then upgraded to SDDM 4. There is already an issue with the option to synchronize from model to data dictionary, in which it only works if you first import any one object from the data dictionary to the model for the other way around to work. I've done this, and now I can succesfully start the synchronization. The thing is, although I have all the objects already created in my database, under a specific schema, everytime I start the synchronization, it always tells me that none of the objects from my model exist on the destination! I already tried defining the owner on the physical model, removing it, changing the connection to connect with the owner of the objects, but nothing seems to work.

You need to clear source stamp information and rename schema in relational model if you want to use sync functionality, Otherwise you can use import from data dictionary wizard (optionally to check "swap target") and to

You can clear source stamp information by selecting all objects on diagram (Ctrl-A) and use "Clear source stamp" in context menu for selected objects.

In next release - there are options to bypass source schema and source object name and you can use different user and change the name of table in physical model in order to synchronize to another schema or even to follow changes in table name.